Uncontrolled microbial growth and activity can create severe optional, environmental, and human safety problems in wastewater treatment and handling systems associated with municipal, industrial and oilfield operations. Problems caused or intensified by microbial growth and activity include corrosion, solids production, and hydrogen sulfide (H.sub.2 S) generation. H.sub.2 S not only has a highly offensive odor, but is toxic even in quite small concentrations. Though H.sub.2 S is present in the air above wastewater in only small concentrations, it has been very difficult to reduce significantly in even the most efficiently operated water handling system.
The microorganisms primarily responsible for H.sub.2 S generation in an anaerobic environment within municipal, industrial and oilfield water operations are sulfate-reducing bacteria. These organisms are ubiquitous and can grow in almost any environment. They are routinely found in waters associated with oil production systems and can be found in virtually all industrial aqueous processes, including cooling water systems, pulp and paper-making systems, chemical manufacturing, and petroleum refining.
Requirements for sulfate-reducing bacteria activity and growth include an anaerobic (oxygen-free) aqueous solution containing adequate nutrients, an electron donor and electron acceptor. A typical electron acceptor is sulfate, which Produces H.sub.2 S upon reduction. A typical electron donor is a volatile fatty acid (e.g., acetic or propionic acids), although hydrogen can also function as an electron donor. Conditions in an oil reservoir subject to seawater flooding are excellent for establishing sulfate-reducing bacteria activity. Seawater contains a significant concentration of sulfate, while connate, or indigenous formation, water contains volatile fatty acids and other required trace nutrients (e.g., nitrogen and phosphorus). Conditions within industrial water systems, such as effluent streams from production operations or cooling water streams, are also conducive to sulfate-reducing bacteria activity due to the anaerobic biofilm which is formed on pipeline, tank or vessel walls. The same is true within the sewers and other piping and facilities associated with municipal wastewater handling systems.
Hydrogen sulfide is corrosive and reacts with metal surfaces to form insoluble iron sulfide corrosion products. In oilfield operations, H.sub.2 S partitions into the water, oil and natural gas phases of produced fluids and creates a number of problems. For instance, oil and gas, which contain high levels of H.sub.2 S, have a lower commercial value than low sulfide oil and gas. Removing biogenic H.sub.2 S from sour oil and gas increases the cost of these products. Hydrogen sulfide is an extremely toxic gas and can be immediately lethal to humans at even small concentrations. Its presence in wastewater systems poses a threat to worker safety. The discharge of produced waters containing high levels of H.sub.2 S into aquatic or marine environments is hazardous because H.sub.2 S reacts with oxygen and lowers the dissolved oxygen levels in the water.
Corrosion caused by sulfate-reducing bacteria and H.sub.2 S frequently results in extensive damage. Pipe systems, tank bottoms and other pieces of equipment can rapidly fail if they have areas where microbial corrosion is occurring. If a failure occurs in a pipeline or storage tank bottom, the released fluid can have serious environmental consequences. If a failure occurs in a high pressure water or gas line, the consequences may be worker injury or death. Any such failure involves substantial repair or replacement costs.
In addition to such industrial problems with the generation of hydrogen sulfide, an altogether too familiar sensation in many communities having wastewater treating systems is the rotten egg smell of the gas hydrogen sulfide. Hydrogen sulfide (H.sub.2 S) is metabolic product of sulfate-reducing bacteria which are ordinarily contained in anaerobic wastewater streams. This gas not only has a highly offensive odor, but is toxic even in quite small concentrations. Though H.sub.2 S is present in the air above wastewater in only small concentrations, it has been very difficult to reduce it significantly in even the most efficiently operated municipal and industrial wastewater treatment systems.
There are two basic approaches to reducing H.sub.2 S in water systems. One approach to the problem of reducing H.sub.2 S odor is to inhibit the production of H.sub.2 S by various means within the water stream. The other approach is to remove the H.sub.2 S produced in such streams.
As to the first method, it is known to add nitrates and/or nitrites to sewage to reduce biological oxygen demand (BOD) and to suppress the formation of H.sub.2 S by bacterial action. (U.S. Pat. Nos. 3,300,404; 4,446,031 and 4,681,687). It is also known to add nitrates to sewage to control objectionable odors (U.S. Pat. No. 4,680,127).
As to the second method, in U.S. Pat. No. 4,680,127 is disclosed a method for removing H.sub.2 S by addition of glyoxal and formaldehyde to scavenge H.sub.2 S from aqueous gas streams. Also, U.S. Pat. No. 4,911,843 to Hunniford et al., discloses a method for removing dissolved H.sub.2 S from sewage by the addition of aqueous solutions of inorganic nitrates to stimulate the growth of bacteria, which consume H.sub.2 S metabolically. A different technique is taught by Tatnall in U.S. Pat. No. 5,500,368. This patent discloses the addition of very small amounts of finely divided particles of anthraquinones to a medium containing sulfate-reducing bacteria to inhibit production of H.sub.2 S therein. U.S. Pat. No. 5,385,844 to Weimer et al., discloses inhibiting H.sub.2 S production by sulfate-reducing bacteria by contacting the medium containing the sulfate-reducing bacteria with a wide variety of anthraquinone compounds.
While all of the foregoing methods for inhibiting the formation of H.sub.2 S and for removing H.sub.2 S are effective to some extent, none is wholly satisfactory because the efficiency of each is low and the relative cost of utilizing them is quite high. For that reason, applicants have sought an efficient and more cost-effective method for keeping the odor level of wastewater streams down to a safe level by a method which involves both keeping the production of H.sub.2 S production low and removing whatever low amount of H.sub.2 S is produced.
Jenneman et al. (Jenneman, G. E., McInerney, M. J. and Knapp, R. M., "Effect of nitrate on biogenic sulfide production", Applied and Environmental Microbiology, 51, No. 6, p. 1205, 1986) determined in laboratory studies that sulfide production was inhibited from sewage sludge, pond sediment and oil field brines upon treatment with nitrate. Their results supported the contention that prolonged inhibition of sulfide production is due to the increase in the redox potential of the environment as a result of the action of nitrate-using bacteria. Two different reasons for this effect were suggested: 1) the redox potential increased due to the buildup of N.sub.2 O or NO or both, which resulted in an oxidized environment, and 2) the levels of sulfate-reducing bacteria decreased during the prolonged exposure to an oxidizing environment and high N.sub.2 O concentration. The results also suggested that short-term inhibition of sulfide production was due to the preferential use of nitrate by the sulfate-reducing bacteria as an electron acceptor, although it was recognized that there was probably a competition among sulfate-reducers and denitrifying (nitrate-using) bacteria present in the waters utilized.
The use of nitrates and denitrifying bacteria to mitigate H.sub.2 S production within an oilfield environment has been well documented. Sublette et al. (Subletter, K. L., Morse, D. E. and Raterman, K. Y., "A field demonstration of sour produced-water remediation by microbial treatment," SPE Production & Facilities, p. 183, August 1994 determined in an oil field pilot study that the addition of a sulfide-tolerant strain of the chemautotroph and facultative anaerobe Thiobacillus denitrificans and nitrate to an aerobic sour brine was able to almost instantly bio-oxidize the sulfide to elemental sulfur and sulfate.
U.S. Pat. No. 5,405,531 to Hitzman et al. discloses that in aqueous systems containing denitrifying bacteria and sulfate-reducing bacterial (SRB), the amount of hydrogen sulfide present can be reduced and the formation of hydrogen sulfide can be prevented by introducing nitrite and nitrate and/or molybdate to the aqueous system. The denitrifying organisms compete with the sulfate-reducing bacteria for the available carbon nutrients, thus preventing the SRB from producing hydrogen sulfide already present in the system. Also taught is that the molybdate and nitrite ions inhibit or kill the sulfate-reducing bacteria. This technology regarding the prevention of hydrogen sulfide production has been referred to as "Biocompetitive Exclusion" (Hitzman, D. O. and Dennis, D. M., "New Technology for Prevention of Sour Oil and Gas", SPE Paper 37908, Paper presented at the SPE/EPA Exploration and Production Environmental conference, Dallas Tex., Mar. 3--5, 1997). The indigenous anaerobic microbial consortium, which includes heterotrophic denitrifiers and anaerobic Thiobacillus species, which includes heterotrophic denitrifiers and anaerobic Thiobacillus species, will rapidly respond to the introduction of alternate electron acceptors such as nitrate, which are more active than sulfate. Their prolific growth results in faster and complete utilization of the volatile fatty acids, thereby denying this carbon source to the sulfate-reducing bacteria. Thus, selective addition of water-soluble inorganic nutrient formulations that include nitrate and nitrite can prevent the deleterious activity of the sulfate-reducing bacteria.
Reinsel, et al. (Reinsel, M. A., Sears, J. T., Stewart, P. S. and McInerney, M. J, "Control of microbial souring by nitrate, nitrite or gluteraldehyde injection in a sandstone column, "Journal of Industrial Microbiology, 17, p. 128, 1996) determined that sulfide production could be inhibited from two different oilfield produced water consortia containing active sulfate-reducing and denitrifying (nitrate-reducing) bacteria in crushed Berea sandstone columns with the addition of nitrate. They attributed this result to the inhibition of sulfate reduction due to presence of nitrite, an intermediate of nitrate reduction, which is a known microbial inhibitory agent. They also determined that the direct addition of very low levels of nitrite inhibit H.sub.2 S production successfully.
While all of the foregoing methods for inhibiting the formation of H.sub.2 S and for removing H.sub.2 S are effective to some extent, none is wholly satisfactory because the efficiency of each is low and the relative cost of utilizing them is quite high. For that reason, applicants have sought an efficient and more cost-effective method for keeping the H.sub.2 S level of wastewater streams down to a safe level by a method which involves both keeping the production of H.sub.2 S production low and removing whatever low amount of H.sub.2 S is produced.